1. Field of the Invention
The invention relates to a direct current voltage boosting/bucking device, more particularly to a direct current voltage boosting/bucking device suitable for driving operation of a high-power light-emitting diode.
2. Description of the Related Art
Current camera phones generally employ a high-power light-emitting diode for supplying light when taking pictures. Hence, a driver circuit is required for driving operation of the light-emitting diode.
FIG. 1 illustrates a conventional driver circuit 1 for a high-power light emitting diode (D). The driver circuit 1 utilizes a direct current voltage boosting chip 11 for boosting an input voltage (Vin) from an input voltage source (such as a battery of a mobile phone) so as to generate an output voltage (Vout) that is higher than the input voltage (Vin) and that is provided to the light-emitting diode (D). It is noted that the input voltage (Vin) must be lower than the output voltage (Vout) in order to ensure normal operation of the direct current voltage boosting chip 11. However, since the voltage range of the input voltage source (e.g., a lithium battery) is usually between 4.2 volts and 3.3 volts, in the case where the input voltage (Vin) is 4.2 volts, the output voltage (Vout) must be higher than 4.2 volts (e.g., 4.3 volts). Therefore, when the light-emitting diode (D) has a working voltage (VF) of 3.2 volts and a working current (ILED) of 700 mA, in order to ensure that only an appropriate portion of the output voltage (Vout) will be present across the light-emitting diode (D) (which only requires 3.2 volts), a resistor (R3) must be coupled in series to the light-emitting diode (D) to bear the excess voltage portion (i.e., 4.3−3.2=1.1 volts). Moreover, in order to enable the direct current voltage boosting chip 11 to generate the fixed output voltage (Vout) (e.g., 4.3 volts), series-connected resistors (R1, R2) must be provided between an output terminal (OUT) and a feedback terminal (FB) of the direct current voltage boosting chip 11, and between the feedback terminal (FB) and the resistor (R3). Therefore, through a feedback voltage that is fed back to the direct current voltage boosting chip 11 and that is set by the resistances of the resistors (R1, R2), the direct current voltage boosting chip 11 is controlled to generate the fixed output voltage (Vout) (e.g., 4.3 volts).
In the aforementioned driver circuit 1, it is noted that a considerable amount of power is consumed by the resistor (R3) such that the power conversion and utilization rates are significantly low. FIG. 2 shows experimental results for the conventional driver circuit 1 to illustrate the power conversion rates for different input voltages (Vin) when the working current (ILED) is 600 mA. It is evident from the data that the power conversion rates decrease with a reduction in the magnitude of the input voltage (Vin). In addition, a change in the working voltage (VF) of the light-emitting diode (D) necessitates corresponding adjustments in the resistances of the resistors (R1, R2, R3) in the driver circuit 1.
FIG. 3 illustrates another conventional driver circuit 2 for a high-power light emitting diode (D). The driver circuit 2 utilizes a synchronous buck-boost driver chip 21 (such as LTC3453) that receives an input voltage (Vin) from an input voltage source (such as a lithium battery) and that performs voltage boosting or voltage bucking so as to generate a suitable output voltage (Vout) for driving the light-emitting diode (D). During operation of the driver circuit 2, the working current (ILED) flowing through the light emitting diode (ILED) is continuously detected. When the working current (ILED) is less than a preset value, a voltage boosting action is conducted to increase the output voltage (Vout) On the other hand, when the working current (ILED) becomes larger than (or at least equal to) the preset value, a voltage bucking action is conducted to decrease the output voltage (Vout). As a result, the working voltage (VF) and the working current (ILED) of the light emitting diode (D) can be maintained at the respective preset value for driving the light-emitting diode (D) to generate a fixed intensity output. FIG. 4 shows experimental results for the conventional driver circuit 2 to illustrate the power conversion rates for different input voltages (Vin) when the working current (ILED) is 150 mA. It is evident from the data that the power conversion rates for the driver circuit 2 are higher than those for the conventional driver circuit 1 of FIG. 1. However, the synchronous buck-boost driver chip 21 requires four MOS transistors (Q1, Q2, Q3, Q4) so as to be able to perform the voltage boosting and voltage bucking operations, which results in higher costs incurred for the driver circuit 2.